The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Internal combustion engine vehicles that employ automatic transmissions typically include a torque converter positioned between the engine and the transmission of the vehicle. A torque converter is a fluid coupling device typically including an impeller at the input coupled to an output shaft of the engine and a turbine at the output coupled to the input shaft of the transmission. The torque converter uses hydraulic fluid to transfer rotational energy from the impeller to the turbine fluidically. The torque converter provides mechanical disengagement of the engine crank shaft from the transmission input shaft, e.g. during vehicle idling conditions to enable the vehicle to stop without stalling the engine.
The rotational speed of the impeller relative to the turbine in the torque converter when fluidically coupled is typically different so that there is a converter slip therebetween. Because large slips between the engine output (torque converter input) and the transmission input (torque converter output) significantly affect the fuel economy of the vehicle, a fluidically controlled torque converter clutch (TCC) may be employed for mechanically transferring torque across the torque converter in fully locked or controlled slip operation. Locking the TCC is generally only used in limited circumstances because of various implications.
A TCC generally has three operating modes—fully released, fully locked, and controlled slip. When the TCC is in the fully released operating mode, all torque transfer across the torque converter is by virtue of fluid coupling with no mechanical torque transfer across the TCC. When the TCC is in the fully locked operating mode, all torque transfer across the torque converter is by virtue of mechanical coupling across the TCC. When the TCC is in the controlled slip operating mode, the slip across the torque converter is controlled by controlling the pressure of hydraulic fluid in the TCC which controls the application force across the TCC.
Rapid changes in throttle request result in rapid changes to engine torque applied to the torque converter. Rapid increases in engine torque can result in torque converter slip changing from a controlled value to an excessive value that must be controlled back to a controlled value.
To increase fuel economy, a desired torque converter slip at the torque converter is minimized However, after a rapid increase in engine torque, for example corresponding to a significant increase in throttle request or heavy tip-in maneuver, the vehicle operator expects an increase in engine speed and torque converter clutch slip. In order to deliver the expected behavior, the TCC pressure must be reduced to a value that allows a timely increase in engine speed yet does not compromise the torque converter clutch hardware by excessively increasing the torque converter clutch slip. Additionally, the increased torque converter slip must subsequently be brought under control after the tip-in maneuver ends in order to reestablish fuel efficient operation.